Pregnancy

rom Egg to Zygote (pp. 1065–1067; Figs. 28.1–28.3)
A. Accomplishing Fertilization (pp. 1065–1067; Figs. 28.1–28.3)
1. Fertilization occurs when a sperm fuses with an egg to form a zygote.
2. Millions of sperm ejaculated into the female reproductive tract are lost due to leakage from
the vaginal canal, destruction by the acidic environment of the vagina, inability to pass the
cervical mucus, or destruction by defense cells of the uterus.
3. In order to fertilize an egg, sperm must be capacitated, a process involving weakening of the
sperm cell membrane in order to allow release of acrosomal hydrolytic
enzymes.
4. When sperm cells bind to the zona pellucida surrounding the egg, they undergo an acrosomal
reaction, in which acrosomal enzymes are released to the oocyte.
a. Hundreds of sperm cells must release their acrosomal enzymes before fertilization can
occur.
b. Once a sperm cell binds to membrane receptors on the oocyte membrane, its
nucleus is pulled into the cytoplasm of the oocyte.
5. Polyspermy, or fertilization by more than one sperm cell, leads to a lethal number of
chromosomes and must be prevented.
a. The entry of a sperm cell into the oocyte causes waves of Ca++ to be released into the
oocyte’s cytoplasm, which activates the oocyte to prepare for the second
meiotic division.
b. The cortical reaction, triggered by the Ca++ surge in the cytoplasm results in
destruction of sperm receptors, preventing other sperm from binding to the oocyte.
c. The slow block to polyspermy involves the material spilled from granulocytes just inside
the plasma membrane that form a swollen membrane that removes other sperm cells from
the surface of the oocyte.
6. After a sperm’s cytoplasmic contents enter an oocyte, its nucleus swells to form the male
pronucleus, as it migrates to the center of the oocyte.
7. Following the entry of the sperm nucleus into the oocyte, the oocyte completes
meiosis II, forming the ovum nucleus, which swells to become the female pronucleus, and
the second polar body.
a. After meiosis II is completed, male and female pronuclei fuse and produce a
zygote, which almost immediately enters into mitosis.

II. Events of Embryonic Development: Zygote to Blastocyst Implantation
(pp. 1067, 1070–1074; Figs. 28.4–28.8)
A. Early Embryonic Development (pp. 1067, 1070–1074; Figs. 28.4–28.8)
1. Early embryonic development begins with fertilization and continues with the movement of
the embryo to the uterus, where it implants in the uterine wall.
2. The mitotic divisions after fertilization occur without much growth between divisions,
resulting in progressively smaller cells, a process called cleavage.
a. Cleavage forms two identical cells, blastomeres, which then form a morula, a
hollow ball of 16 or more cells, by 72 hours.
b. After 4–5 days, the blastocyst, a fluid-filled hollow sphere consisting of a single layer of
trophoblast cells and an inner cell mass, escapes from the degrading zona
pellucida to implant in the uterine wall.
3. Implantation occurs after 6–7 days; the trophoblast adheres to the endometrium and produces
enzymes that irritate the endometrium.
a. Uterine capillaries become permeable and leaky, and the trophoblast proliferates, forming
the cytotrophoblast and the syncytiotrophoblast.
b. Trophoblast cells secrete human chorionic gonadotropin (hCG), which acts on the corpus
luteum to maintain its presence until the placenta can adequately support the developing
fetus.
4. Placentation is the formation of the placenta and is the process of proliferation of the
trophoblast.
a. The placenta is fully functional as a nutritive, respiratory, excretory, and endocrine organ
by the end of the third month of gestation.
III. Events of Embryonic Development: Gastrula to Fetus (pp. 1074–1081;
Figs. 28.9–28.14)
A. Formation and Roles of the Extraembryonic Membranes (pp. 1074–1075)
1. While implantation is occurring, the blastocyst is being converted into a gastrula,
in which three primary germ layers form and embryonic membranes develop.
a. The amnion forms the transparent sac ultimately containing the embryo and
provides a buoyant environment that protects the embryo from physical trauma.
b. The yolk sac forms part of the gut, produces the earliest blood cells and blood
vessels and is the source of germ cells that migrate into the embryo to seed the
gonads.
c. The allantois is the structural base for the umbilical cord that links the embryo to the
placenta and becomes part of the urinary bladder.
d. The chorion helps to form the placenta and encloses the embryonic body and all other
membranes.
B. Gastrulation: Germ Layer Formation (pp. 1075–1076; Fig. 28.9)
1. Gastrulation is the process of transforming the two-layered embryonic disc to a three-layered
embryo containing three germ layers: ectoderm, mesoderm, and endoderm.
2. Gastrulation begins with the appearance of the primitive streak, which establishes the long
axis of the embryo.

a. The endoderm gives rise to epithelial linings of the gut, respiratory, and urogenital
systems, and associated glands.
b. The mesoderm gives rise to all types of tissues not formed by ectoderm or
endoderm, such as muscle tissue.
c. The ectoderm gives rise to structures of the nervous system and the epidermis.
C. Organogenesis: Differentiation of the Germ Layers (pp. 1076–1080; Figs. 28.10–28.14)
1. Organogenesis is the formation of organs and organ systems; by the end of the
embryonic period at 8 weeks, all organ systems are recognizable.
a. Chemical signals from the notochord induce neurulation, the formation of the brain and
spinal cord, in the first event of organogenesis.
b. As the embryo develops from the neural plate, the ectoderm folds into the neural tube,
which gives rise to the brain at the anterior end and the spinal cord from the rest.
c. Neural crest cells migrate and give rise to the cranial, spinal, and sympathetic
ganglia, adrenal medulla, pigment cells of the skin, and some connective tissues.
d. The neural tube folds from both ends toward the center, forming a tube lined with
endoderm, the primitive gut, which quickly gives rise to organs of the GI tract.
e. Mesodermal specialization forms the notochord and gives rise to the dermis,
parietal serosa, bones, muscles, kidneys, gonads, cardiovascular structures, and connective
tissues.
f. By 31⁄2 weeks, the embryo has a blood vessel system and a pumping heart.
g. Unique prenatal vascular modifications include umbilical arteries and veins that carry
blood to and from the placenta, a ductus venosus that serves to bypass the
liver, and the foramen ovale and ductus arteriosus, used to divert most blood
flowing through the heart away from the pulmonary circulation into the systemic
circulation.
IV. Events of Fetal Development (pp. 1081–1082; Fig. 28.15; Table 28.1)
A. The fetal period extends from weeks 9–38 and is a time of rapid growth of body
structures established in the embryo (p. 1081; Fig. 28.15).
B. During the first half of the fetal period, cells are still differentiating into specific cell types to
form the body’s distinctive tissues (pp. 1081–1082).
V. Effects of Pregnancy on the Mother (pp. 1082–1085; Fig. 28.16)
A. Anatomical Changes (pp. 1082–1083; Fig. 28.16)
1. The female reproductive organs and breasts become increasingly vascular and
engorged with blood.
2. The uterus enlarges dramatically, causing a shift in the woman’s center of gravity and an
accentuated lumbar curvature (lordosis).
3. Placental production of the hormone relaxin causes pelvic ligaments and the pubic symphysis
to soften and relax.
4. There is a normal weight gain of around 28 pounds, due to the growth of the fetus,
maternal reproductive organs, and breasts, and increased blood volume.
B. Metabolic Changes (p. 1083)

1. As the placenta enlarges, it produces human placental lactogen (hPL), which works with
estrogen and progesterone to promote maturation of the breasts for lactation.
2. Human placental lactogen also promotes the growth of the fetus and exerts a
glucose-sparing effect on maternal metabolism.
3. Plasma levels of parathyroid hormone and activated vitamin D rise, ensuring a positive
maternal calcium balance throughout pregnancy.
C. Physiological Changes (pp. 1083–1085)
1. Many women suffer morning sickness during the first few months of pregnancy, until their
systems adapt to elevated levels of hCG, estrogens, and progesterone.
2. Heartburn often results from the displacement of the esophagus, and constipation may result
due to the decreased motility of the digestive tract.
3. The kidneys produce more urine, because maternal metabolic rate is higher, and there is
additional fetal metabolic waste that must be eliminated.
4. Vital capacity and respiratory rate increases, but there is a decrease in residual volume, and
many women suffer from difficult breathing, or dyspnea.
5. Blood volume increases to accommodate the needs of the fetus and may increase up to 40%
by the 32nd week of pregnancy.
6. Mean blood pressure decreases during the second trimester, but then returns to normal levels
during the third trimester; cardiac output increases by 35–40%.
VI. Parturition (Birth) (pp. 1085–1087; Figs. 28.17–28.18)
A. Parturition is the process of giving birth and usually occurs within 15 days of the
calculated due date, which is 280 days from the last menstrual period (p. 1085).
B. Initiation of Labor (p. 1085; Fig. 28.17)
1. Estrogen levels peak, possibly due to rising levels of fetal adrenal cortical hormones
(cortisol), stimulating myometrial cells of the uterus to form abundant oxytocin
receptors and antagonizing the quieting effect of progesterone on uterine muscle.
2. Fetal cells produce oxytocin, which promotes the release of prostaglandins from the placenta
and further stimulates uterine contraction.
3. Increasing cervical distention activates the mother’s hypothalamus, which signals the release
of oxytocin, setting up a positive feedback loop in which further distention of the cervix
promotes the release of more oxytocin, which causes greater contractile force.
4. Expulsive contractions are aided by a change that occurs in an adhesive protein, fetal
fibronectin, converting it to a lubricant.
C. Stages of Labor (pp. 1085–1087; Fig. 28.18)
1. The dilation stage of labor extends from onset of labor to the time when the cervix is fully
dilated by the baby’s head, at about 10 cm in diameter.
a. At first, only the superior uterine muscle is active, but as labor progresses, contractions
become more vigorous and rapid, and more of the uterus becomes involved.
b. As the infant’s head is forced against the cervix, the cervix softens, thins, and
dilates; ultimately, the amnion ruptures, releasing amniotic fluid.
c. The dilation stage is the longest part of labor; during this phase, the infant’s head becomes
engaged as it passes into the true pelvis, and as descent continues, the

baby’s head rotates so that its greatest dimension is along the anteroposterior line,
allowing easier navigation of the pelvic outlet.
2. The expulsion stage extends from full dilation until the time the infant is delivered.
a. Crowning occurs when the baby’s head distends the vulva, and once the head has been
delivered, the rest of the baby follows much more easily.
b. When the baby is in the vertex, or head-first, presentation, the skull acts as a wedge to
dilate the cervix.
c. After birth, the umbilical cord is clamped and cut.
3. During the placental stage, uterine contractions compress uterine blood vessels,
limiting bleeding, and cause detachment of the placenta from the uterine wall,
followed by delivery of the placenta and membranes (afterbirth).
VII. Adjustments of the Infant to Extrauterine Life (p. 1087)
A. The Apgar score is an assessment of the infant’s physiological status based on heart rate,
respiration, color, muscle tone, and reflexes; a score from 8–10 indicates a healthy baby.
B. Taking the First Breath and Transition
1. Once the placenta is no longer removing carbon dioxide from the blood, it builds up in the
infant’s blood, resulting in acidosis that signals the respiratory control centers.
a. The first breath is very difficult, due to the fact that airways are small, and the lungs are
collapsed.
2. The transitional period is the 6–8 hours after birth characterized by intermittent
waking periods in which the infant’s heart rate, respiratory behavior, and body
temperature fluctuate.
C. Occlusion of Special Fetal Blood Vessels and Vascular Shunts
1. After birth, the umbilical arteries and veins constrict and become fibrosed, becoming the
medial umbilical ligaments, superior vesical arteries of the bladder, and the round ligament of
the liver, or ligamentum teres.
2. The ductus venosus closes and is eventually converted to the ligamentum venosum.
3. The pulmonary circulation becomes functional, and pressure in the right side of the heart
decreases, while pressure in the left side increases, causing pulmonary shunts to close.
a. A flap of tissue covers the foramen ovale, ultimately sealing it and becoming the fossa
ovalis, while the ductus arteriosus constricts, becoming the ligamentum
arteriosus.